The paper presents a study of acoustic spectral adaptation in speech based on the material of dialogues from the corpus of Russian dialogue speech SibLing. The corpus consists of dialogues between pairs of twins, brothers, sisters among themselves, with close friends, strangers of the same sex, strangers of the opposite sex, strangers of higher rank and greater age. To analyze the differences and similarities between speakers, long-term average speech spectra (LTASS) were calculated. The formant characteristics of the vowels for all speakers were calculated using the method for determining the characteristics of the transfer function of the vocal tract. The results for LTASS show that the correlation in the spectra of dialogues with a friend was the highest. The correlation of LTASS in dialogues a stranger of the opposite sex was the lowest. In most cases, the correlation of the average spectra was higher from the beginning to the end of the first task, but dropped significantly in the second task. The average values of the Euclidean distance for the formants and the analysis of the formant pictures of vowels showed that in most cases there is a mutual adjustment of the interlocutors in the process of dialogue.